Silver has long been valued as a precious metal, and it is used to make ornaments, jewelry, high-value tableware, utensils (hence the term silverware), and currency coins. One of the drawbacks of silver is its tendency to tarnish naturally in atmosphere due to the formation of silver sulfide. The appearance of the silver rapidly deteriorates based upon the thickness of the silver sulfide coating from initial yellowing of the surface finally resulting in a black film. The most common tarnish causing elements are food (onions, eggs, mayonnaise, salad dressing, salty foods) salt, wool, felt, rubber bands, latex gloves, carpet padding, sulfur in the air and oily residue from human hands and fingers.
Tarnish is generally caused by a reaction of sulfur, either from the surrounding air or perspiration that reacts with silver to form Ag2S. Several species other than silver sulfide can also be found on tarnished surfaces. These include sulfate, chloride, oxide, organic carbon, and oxygenated organic carbon species and a carbonate or carbonyl. In some cases these species can be present at comparable or very much higher concentrations than silver sulfide. Particulate deposition can result in species such as sodium, potassium and silicon being present in the tarnished layer. The relative concentrations of each of these species will obviously vary, depending upon the environment to which the silver surface is exposed. Even if handled carefully, and exposed in closed cases, there is sufficient sulfur and other of the above species in most air to result in tarnishing of sterling silver after a few days, which appears as a black scale. To combat this problem, jewelry stores and others use cloth or similar strips treated to function as a sulfur “getter” in display cases, lengthening the time before which noticeable tarnishing appears. The sulfur “getter” includes a substance with a higher chemical reactivity than silver to remove sulfur based chemicals from the air. Such a treatment, however, is not possible for sterling silver kept in environments that are not well sealed. Replacement of the sulfur absorption strips is also required after some time.
Tarnish of silver can be removed by either polishing removal of the tarnish film or by reversing the chemical reaction that caused formation of silver sulfide. Whatever the method employed, tarnish removal is a laborious process that may damage the appearance of the silver article.
There are various methods practiced currently that minimize or attempt to prevent the formation of tarnish. Of the many approaches that have been taken to solving this problem in the past, most of these rely upon alloying of silver to make more tarnish resistant compositions.
At least five different approaches to alloying of silver to reduce the onset of tarnishing have been made:
U.S. Pat. No. 4,775,511, issued October 1988, describes the use of additions to silver-copper, silver-gold and silver-copper-gold alloys of at least one element of the following: Cr, Ta, Al, Ti, or Th, where the added amounts of these elements does not exceed 1.5 wt. % as a substitute for silver. These elements were found to form a thin oxide layer, which was stable and did not affect the properties of the silver based alloy. The elements were found to be self-healing, forming a layer of oxide and also reacting preferentially with sulfur to form sulfides, rather than formation of silver sulfide. In this way, tarnish resistance was improved. All of these elements were recognized as having heats of sulfide formation, which were higher than that of silver. The preferential additions to silver-copper alloys were Al and Cr at about 0.5 wt. % and for silver-gold and silver-copper-gold alloy Cr at 0.4 wt. %. An improvement of 10-12 times increase in sulfide tarnish inhibition for sterling silver was claimed. This increase could also be reached by adding 0.75% Ti, 1.25% Th or 1.5% Ta.
U.S. Pat. No. 5,817,195 describes a tarnish resistant alloy containing 6.57% Zn, 0.25% Cu, 0.25% Ni, 0.25% In, and 0.18% metal silicates. These silicates involve other alloying elements than those listed, because those listed are not silicate formers. However, it is claimed that these silicates improve castability by making the molten metal more fluid. This alloy is claimed to have a more stable color than sterling silver. The addition of nickel eliminates the brittleness of traditional sterling silver after casting. Zinc replaces copper to enhance tarnish resistance and improve overall corrosion resistance. Nickel must be added to the alloy in equal amounts with copper to cause all the constituents to remain in solution.
U.S. Pat. No. 5,039,479 describes the usefulness of the additions of silicon, boron and tin to an alloy similar to that described in U.S. Pat. No. 5,817,195. The preferred composition is 1.85% Zn, 0.05% In, 4% Sn, 1.44% Cu, 0.01% B, and 0.05% Si. Si is added as a deoxidizer and is claimed to reduce the porosity of recast alloys. B is added to reduce the surface tension of the molten alloy. Zn is added to reduce the melting point of the alloy, and whiteness to act as a copper substitute, to help in deoxidization and to improve alloy fluidity during casting. Sn is added to improve tarnishing resistance and for its hardening effect. In is added as a grain refining agent and improves the wettability of the alloy. The composition described is sufficiently pure in silver to qualify as sterling silver. A similar approach is taken by U.S. Pat. No. 5,882,441 which describes a tarnish resistant 4.5% Zn-2.9% Cu-0.1% S alloy.
Addition of more noble elements increases the cost of sterling silver, but is highly effective. U.S. Pat. No. 5,037,708 describes an alloy containing 5% Pd, 2% Cu and 0.5% In or Zn. In this alloy the Cu has been replaced by Pd to enhance tarnish resistance and corrosion resistance and also to improve color stability. The working and casting properties are also improved, and Cu and In also help reduce brittleness.
The hardness and tarnish resistance of fine silver, with at least 99.5% purity, is claimed to be improved by the disclosure of U.S. Pat. No. 6,139,652, issued Oct. 31, 2000. Small additions of Al, Sb, Ca, Ga, Ge, In, Li, Mn, Mg, Si, Sn, Ti, or Zn are added. The cast alloy is then annealed in an oxygen-rich atmosphere to give internal oxidation, hardening the alloy. This allows it to be age hardened to at least 136% of its annealed hardness, improved tarnish resistance is also claimed, likely by formation of surface oxides rich in the alloying elements.
Several of these alloys have been commercialized. For example, Ney Paliney 6 is a platinum silver-based alloy used for throttle position sensors, guidance systems, potentiometers, trimmers, communications and bar code readers. Sterling “D” is a white colored sterling silver offered by United Precious Metal Refining. It claims to have excellent tarnish resistance.
The addition of Ge to sterling silver is also noted to reduce tarnishing and fire stain. This alloy is a cadmium-free alternative to the 2 and 4% cadmium bearing grades that have been used in the past. This composition is disclosed by UK Patent 2,255, 348B. A similar composition was claimed by Metaleurop in their German application 4,213,897 of Nov. 5, 1992. This alloy contains between 0.5 and 3% Ge, the balance of Cu to give 7.5% alloy addition and 92.5% Ag to give sterling silver. This alloy is currently marketed as Argentium Silver. The common perception is that Argentium silver is tarnish resistant but in fact this alloy also tarnishes similar to regular sterling silver, the only difference being that the color of the tarnish layer is transparent yellow instead of the black layer developed in regular silver. The cost of Argentium silver alloy is almost double the cost of regular sterling silver providing another drawback for its use by consumers.
None of above methods, consisting of alloying additions, completely eliminates tarnishing of silver and additionally is not cost effective as compared to the most common form of silver which is the sterling silver.
Metallic coatings by electroplating, such as thin (150 Å) transparent coatings of Ni, Rh, Pt, Ir and Pd have also been used [3]. In addition electroplated silver alloy coatings containing Cd, Sb, Sr and Pd are possible. The application of these coatings is complicated and expensive, and additionally involves the use of corrosive chemicals. When these platings are too thin (due to cost considerations) they easily wear off causing localized tarnishing of silver to develop.
Oxide coatings of Al, Be, Zr, Mg, Ti, and Nb can be produced by sputtering or cathodic reduction of solutions containing the metal ions [1, 2]. These are expensive processes and it is complicated to produce deposits on curved articles. In addition, the abrasion resistance of these coatings is poor.
Chromate conversion coatings are cheap, easy to perform, and provide relatively good corrosion resistance. These coatings can be applied by either chemical immersion or electrochemical treatment. One process has been patented that uses phosphate with 1-2% chromate followed by drying at 150° C. [2]. Electrochemical chromate coatings are more expensive to produce than those made by the immersion systems. All of the chromate processes use hexavalent chromium which is toxic and in many countries its use is either being banned or phased out.
Non-chromate conversion coatings are generally based on tin compounds that produce coatings that are not as durable as chromate coatings [2, 4-6]. There are several coatings based on organic coating materials, which provide hydrophobic films on silver that are resistant to tarnishing in accelerated tests. In some cases, such as when the coating s are based on thiols, the stable complex of silver is formed but in no case are these coatings as durable as chromate conversion coatings.
Organic coatings protect silver by forming a physical barrier between the sulfur and the silver, and are generally used for storage and display of silver items. Lacquers can be applied by brushing, dipping or spraying, and organic inhibitors can be added to improve protection. The main problem with these layers is that they are nearly always visible and if they are too thin the silver items are not sufficiently protected.
Polishes have been produced that claim to give tarnish protection and contain reducing compounds such as sodium dithionite and corrosion inhibitors such as morpholine [6]. There are no data known to the Applicant on the extent of protection afforded.
Other methods of tarnish prevention include storing silver in special storage materials such as certain polymers that react with sulfur to prevent its attack on silver [7]. This type of method helps keep the silver in a reasonable condition only so long as it is kept within the storage materials.
The use of atomic layer deposition (ALD) technique as in bibliography entry [8] provides for the most conformal technique thus eliminating any non-uniformity that can cause discoloration of silver due to optical interference. The method described in [9] and commercialized by the Beneq Oy company under the nSILVER® process is practiced in a jewelry context solely for the purpose of allowing the display of silver products in retail stores without the need for storing in special storage materials. Silver products coated with oxides using an ALD process have limitations due to change in appearance of the silver depending on the type of oxide and the coating thickness as well as loss of ability to prevent tarnishing over time due to wearing out of the protective layer or localized breakdown of the protective layer due to adhesion problems.
In India, a style of jewelry making called “Kundan” dates back several thousand years. Kundan jewelry work involves the use of diamonds or cut glass, various precious and semi-precious colored stones, pure 24K gold as well as pure silver and detailed enameling work. One method of Kundan jewelry manufacturing uses silver foil that is cut to a shape similar to the pavilion of the uncut diamonds or glass pieces. These silver foil shapes are placed in the skeletal structure of the jewelry piece which is fashioned such that there are holder or cups for receiving the stones. The uncut diamonds or cut glass pieces are placed over the foil in order to give a luminous and brilliant appearance. The stone is secured by using 24K gold foil which is beaten to conform to the periphery of the stone. Over a period of time the embedded silver foil gets tarnished causing the Kundan piece to get discolored.
Gold has been a highly sought-after precious metal, even more so than silver, for coinage, jewelry, and other objects since the beginning of recorded history. Like other precious metals gold is measured by troy weight and grams. When alloyed with other elements the term carat or karat is used to indicate the amount of gold present. The price of gold being several orders of magnitude higher than silver makes gold's use in jewelry is the key application other than its use as an investment or in other industries. The price of gold has been steadily rising over the decades making its use in jewelry more and more expensive. There has long been a great demand for gold plated products and jewelry which occupy a large segment of the low-cost decorative articles market. Gold platings are applied on silver or other metals by chemical or electrochemical means to provide the object with the appearance of carat gold. The plating thickness varies according to the value placed on the article. Gold platings wear off with time and, when plated on silver, result in slow diffusion of silver to the surface causing the gold color to fade and tarnishing to occur. The high price of gold in the current economy has made gold electroplated articles and jewelry more expensive than previously.
There are methods to create the appearance of gold by vapor deposition of titanium nitride coatings but even these require the coating to contain some percentage of gold to give to a carat gold appearance. The watch industry has used titanium nitride/gold vapor deposited coatings as a means to produce low cost gold colored watches. Additionally these physical vapor deposition (PVD) based methods also require barrier layers to be deposited beneath the titanium nitride/gold layer by electroplating such as Nickel plating or Nickel with Palladium flash platings on brass before the PVD process. The watch industry uses electroplating to first deposit a layer of nickel or nickel-palladium alloy. The nickel based base layer is overlain with a PVD coating of TiN followed by gold deposition by PVD to achieve a gold appearance. Such methods for providing a cheaper alternative than gold plating do not last long under use conditions in watches, jewelry and the like. The gold appearance wears off and/or the object becomes discolored due to environmental corrosion.
There are methods of providing gold colored coatings on metals typically used by the finishing industry (door hardware, etc.) by the use of lacquer coatings but such coatings are easily distinguished from gold due a wet, shiny look. Such coatings are typically applied in thickness ranging from a few microns to tens of microns depending on the application. They cannot be applied on complex fine geometries due to the flow characteristics of the lacquer which cause loss of suppleness. Lacquer coatings can be applied in any color or as clear coatings and have great use in the automobile and hardware industry. When used on jewelry, however, they impart the look of a cheap imitation gold color which does not match the color of carat gold. Further lacquer coatings may impart an unnatural look and feel to the product. Additionally most lacquer coatings can be easily stripped off by common solvents such as acetone.